Purpose:
It has been proven that bifocal lenses designed with relative positive additions can slow ocular elongation and thus myopia progression. This study addressed a related question of whether similar designed lenses with relative negative additions can control hyperopia using young chicks as an animal model.

Methods:
To induce hyperopia, chickens wore monocular +5 diopter (D) single vision lenses (SVLs) from 7 days of age for 3 days; the lenses were then switched for either +10 D SVLs or 2-zone concentric bifocal lenses (BFLs), which were worn for 5 days. BFLs had a central zone power of +10 D and one of 3 peripheral zone powers (plano, +5 or +8 D, corresponding to additions of -10, -5 or -2 D, respectively). For all BFL designs, both 2.5 and 4.5 mm central zone diameters (CZDs) were tested. Central refractive errors and ocular axial parameters were measured using static retinoscopy and high frequency A-scan ultrasonography.

Results:
At the last time point, the control group (i.e., wearing +10 D SVLs) was most hyperopic (+9.37 D), with the group wearing 2.5 mm CZD BFLs with the highest (-10 D) addition being least hyperopic (+4.28 D). For both the 2.5 and 4.5 mm CZDs, there were trends towards decreasing induced hyperopia with increasing negative add power, with this dose effect being significant for 2.5 mm CZD lenses. Induced changes in vitreous chamber depth and optical axial length (relative shortening) as well as choroid thickening followed trends consistent with induced refractive errors.

Conclusions:
Our study explored the possible application of BFLs as a treatment to control hyperopia. Our data provide evidence that 2-zone concentric BFLs incorporating peripheral negative additions could restrain lens-induced hyperopia progression in young chicks, and treatment effects increasing with both add power and larger peripheral zones. Further studies are warranted to examine whether mammals and primates show similar beneficial effects.